Electric wave translating device



April 6, 193 7.

E. B. PAYNE ELECTRIC WAVE TRANSLATING DEVICE Qriginal Filed May 11, 19553 Sheets-Sheet l F/GZA CA L IBRA TE 0 4 TTENUA TOR C C mm UNKNOWNNETWORK INVENTOR E B. PAYNE April 6, 1937. E. B. PAYNE ELECTRIC WAVETRANSLATING DEVICE Original Filed May 11, 1935 3 Sheets-Sheet 2 F Q. l 62V! 9 a *II a. aJET n 51 5 2/ Z a w o g 3 Q P 8 WW b m n M a m AUTOMATIC(Pl/.0 r WIRE on PILOT emu/v51.) cam CO/VTPOL FIG. /Z.

E m A WP W5 E P um "-2 L L E Q MIA". 1? 2/ HI A T TORNEV April 6, 1937.E. B. PAYNE ELECTRIC WAVE TRANSLATING DEVICE 3 Sheets-Sheet 3 OriginalFiled May 11, 1935 c m MMF 8 55 6O 62 64 66 68 7O 5 M 5 a 5 O 5 FIG. /4

nvvavrop E. 8. PA YNE' BY/ ATTORNEY Patented Apr. 6, 1937 UNITED STATESPATENT OFFICE ELECTRIC WAVE TRANSLATING DEVICE Edward B. Payne, NewYork, N. Y., assignor to Bell Telephone Laboratories, Incorporated, NewYork, N. Y., a corporation of New York 4 Claims.

This application is a division of application Serial No. 20,951, filedMay 11, 1935, for Electric wave translating systems.

This invention relates to electric Wave translating devices.

Objects of the invention are to control wave amplitude and phase.

Further objects are to control network attenuation and impedance, and itis also an object of the invention to control relations between networkattenuation and impedance, as for example the relation of imageimpedances of adjustable attenuators to their attenuation variation.

In one specific aspect the invention is a threeterminal, three-capacityattenuating network, for example a T or 1r network of condensers, withmeans for relatively moving the plates or armatures of each of thecondensers simultaneously to vary the attenuation of the network, thecondensers having their armatures shaped to produce a prescribedrelation between the attenuation of the network and its imagecapacities, for instance to cause the image capacities to remainconstant as the attenuation varies.

Other objects and aspects of the invention will be apparent from thefollowing description and claims.

Figs. 1 to 6A show three-way variable condenser units embodying forms ofthe invention, Figs. 1 to 3A being T-networks and Figs. 4 to 6A being 1rnetworks;

Fig. '7 shows schematically an embodiment of the invention in atransmission loss measuring circuit;

Figs. 8 to 10 show schematically three embodiments of the invention intuned amplifiers;

Figs. 11 and 12 show embodiments of the invention in transmissionequalizing systems; and

Figs. 13 and 14 show curves illustrating a method of determiningsuitable shapes for condenser plates.

In Fig. 1, a T-network of capacities Ca, Oh and Co forms a variableattenuator for waves transmitted therethrough, for example, fromterminals l and 2 to terminals 3 and 2. The network is shown terminatedin its image capacities C01 and C02, a source of voltage e beingindicated in series with the terminating capacity Cor.

Capacity Ca comprises plates or armatures a1 and ad, which may berelatively movable for varying the capacity Ca; capacity Cb comprisesarmatures b2 and be, which may be relatively movable for varying thecapacity Cu; and capacity Cc comprises armatures c3 and ca, which may berelatively movable for varying the capacity Co. The portion of theT-network at the potential of plates ad, bd and cd is designated d.Means may be provided, for example, as indicated in Figs. 2 and 2A orFigs. 3 and 3A described hereinafter, for varying the capacity Ccsimultaneously with and in opposite sense to the capacities Ca and Cb,the capacity Cc increasing when Ca and Cb decrease, and the capacity Ccdecreasing when Ca and Cb increase. In accordance with the invention,the condenser plates, though shown as segments of circles for the sakeof simplicity and clearness, may be shaped, as explained hereinafter, togive the following properties:

(1) Image capacities at each end of the network independent of thesetting of the plates associated with attenuation which varies with theplate setting, (1. e., image capacities independent of attenuationvariation);

(2) The image capacities independent of the plate setting, as in (1) anddifferent from each other; and

(3) Image capacities and attenuation which are both dependent in anydesired way on the plate setting.

These properties may be obtained by designing the attenuator, asexplained hereinafter, in accordance with formulae which relate thenetwork capacities Ca, Cb and Ge with its image capacities CO1 and C02and its attenuation constant 0, which is the real part of the transferconstant, the imaginary part being zero. These formulae are:

C /F sinh 0 Here 0 is expressed in nepers. The significance of imagecapacity is apparent from the relation,

where Z: is image impedance and CI is image capacity. When Ca=Cb theimage capacities C01 and C02 are equal and the T is symmetrical.Networks designed to have the property (1) or the property (2) abovehave the important property:

(4) The network exhibits a constant capacity at one end independent ofplate setting when the other end is terminated in its image capacity.

The variable attenuator of Fig. 1 may be realized for example in thestructure shown in Figs. 2 and 2A, which are, respectively, a frontelevation and a right side or right end elevation of a condenserembodying the capacities Ca, Cb and Co. Plates an, 132 and 03 are fixedor stationary. S is a shaft rotatable in opposite directions asindicated by the curved arrows in the figure. The shaft 5 carries platesas, be and ca, these four elements being direct electrical connectionand at the same potential. When the shaft rotates plate us toward or toincrease capacity Ca, it also rotates plate bd toward hi to increasecapacity Cb and rotates plate Cd away from 03 to decrease the capacityof (3c.

The shape of the movable plates that is suitable for obtaining a desiredrelation between angle of rotation e, in degrees, and insertion loss ofthe attenuator, in decibels, can be obtained with the aid of Equations Ito III which relate capacity and loss. Consider for example the simplecase in which the desired relation between and decibel loss is linear,from zero loss at :l80 to 4.8 decibel loss at =O, the image impedancesbeing C01=5O micro-microfarads and 002: micro-microfarads. Curve II ofFig. 13 represents the relation between angle and loss in such case.Curve L, showing the relation between the attenuator loss and thecapacity of one of the elements or condensers, say Ca, of theattenuator, is computed, the formula for Ca for example being I.Assuming Ca. made up of a 50 micro-microfarad fixed capacity plus avariable air condenser whose capacity will be designated Ca, the valuesof Ca for different angles are given by the relation Ca:Ca-50 so valuesof Ca corresponding to angles 30", 45, 60 can be obtained from thecorresponding values of Ca given by the curve L for these angles. Thesevalues of and Ca are tabulated in the first two columns of the tablebelow. The differences between successive values of Ca are designated ACand are tabluated in the third column; and these values of AC areemployed as indicated below for obtaining the fourth column of thetable.

(degrees) Os AC r/l;

30 71 1. 646 45 l. 20 .40 l. 934 60 l. 85 65 2. 228 75 3. 00 1.15 2. 96490 4. 50 1.50 3. 385 7. (l0 2. 50 4. 370 10. 6 3. 6 5. 244 15. G 5. O 6.18 20. G 5. 0 6. 18

The capacity of a 15 sector of set of circular disks of radius 1' is er/24W where 70 is a constant depending on the separation of the platesand the dielectric constant or" the medium, assumed unity for an aircondenser; so, from the successive values of AC, the radii of successivesectors of the air condenser can be computed by the formula:

The polar curve corresponding to curve N is curve 0 in Fig. 14, whichshows the shape of the movable plate ad of condenser Ca, the plates inbeing, for example, semi-circular with a radius at least as lar e as themaximum radius of plate as.

The shapes of the movable plates of condensers Cb and Co can bedetermined in the manner indicated above for determining the shape ofplate on, the Formulae II and III being used in the case of Cb and Cc,respectively, instead of the Formula. I.

The curves H, L, M, N and 0 show the relations between the angle ofrotation, loss, capacity of one of the elements of the attenuator, andradius (except for a constant) of the movable air condenser giving thecapacity, for a particular case. The same procedure can be followed formore complicated cases, as for example when the less is not to belinearly proportional to the angle of rotation.

The general case will now be considered, for derivation of a generalformula for the radius. An elementary area of a condenser rotor plate isand the element of capacity of a set of similar plates is Kp dqa where Kis a constant. This element or" capacity must equal dC=I"(0)d(p if C f(p) expresses the relation between capacity and angle.

Then

However, the relation between capacity and angle is given indirectlythrough Equations I to III connecting capacity and loss, and the de-This gives a means of computing p for various values of the angle q) ifthe functional relations are known. The graphic procedure illustratedabove may be applied where these relations are unknown or give toocomplicated form to Equation IV.

Another example of a structure realizing the attenuator of Fig. 1 isshown in Figs. 3 and 3A, which are, respectively, a left side or leftend elevation and a front elevation of a condenser embodying thecapacities Ca, Cb and Cd. Plates a1, b2 and c3 are fixed or stationary,and correspond respectively to plates on, b2 and c3 of Figs. 1, 2 and2A. S is a shaft rotatable in opposite directions, and corresponds toshaft S of Figs. 2 and 2A. The shaft S carries two plates cl, thesethree elements being in direct electrical connection and at the samepotential; and these three elements correspond to portions (1, an, beand ca of Figs. 1, 2 and 2A. When the shaft S rotates plates at towardan and 222' to increase Ca. and increase Cb, it also rotates the platesd away from plates oz, to decrease the capacity Cc. The use of twoplates d (or a thick plate),

instead of a single thin plate, tends to reduce the direct capacitybetween plate 411 and In. If desired, either of the two plates as can beomitted.

Fig. 4 shows acircuit corresponding to that of Fig. 1 except that theT-network is replaced by a r network of capacities CA, CB and Cocorresponding to the capacities CA, Cb and Ge and forming an attenuatorsimilar to that of Fig. 1.

Capacity CA comprises plates or armatures A1 and A2, which may berelatively movable for varying the capacity CA; capacity CB comprisesarmatures B1 and B2, which may be relatively movable for varying thecapacity C13; and capacity Cc comprises armatures C2 and C3, which maybe relatively movable for varying the ca pacity C3. Means may beprovided, for example as indicated in Fig. 5 or in Figs. 6 and 6Adescribed hereinafter for varying the capacity C, simultaneously withand in opposite sense to the capacity CB and Co. In accordance with theinvention the condenser plates may be shaped to give the above-mentionedproperties (1), (2), (3) and (4). These properties may be obtained bydesigning the attenuator in accordance with the following formulae:

C w cmca A sinh 0 C 01 1/ 01 n2 B tanh 0 sinh 0 C 02 o1 02 C tanh 0 sinh0 When CBICC the image capacities C01 and C02 are equal and the Ir issymmetrical.

The variable attenuator of Fig. 4 may be realized for example in thestructure shown in Fig. 5, which is a front elevation of a condenserembodying the capacities CA, CB and C0. Plates B1, C2 and A1 are fixedor stationary. S1 and S2 are electrically conducting sections of a shaftwhich are separated by an. insulating section S3, the shaft beingreversibly rotatable. The section S1 carries plates B3 and Ca, which areconnected to terminal 3; and the section S2 carries plate A2, which isconnected to terminal 2. When the shaft rotates plate A2 toward A1 toincrease capacity CA, it also rotates plate B3 away from B1 to decreasecapacity Cs and rotates plate C3 away from C2 to decrease capacity Cc.

Another example of a structure realizing the attenuator of Fig. 4 isshown in Figs. 6 and 6A, which are, respectively, a left side or leftend elevation and a front elevation of a condenser embodying thecapacities CA, CB and Co. Plate A1, B1 is connected to terminal I,corresponds to plates A1 and B1 of Figs. 4 and 5, and is fixed orstationary; and plate A2, C2 is connected to terminal 2, corresponds toplates A2 and C2 of Figs. 4 and 5, and is fixed or stationary. S4 is ashaft rotatable in opposite directions, and carries plate B3, C3, whichis connected to terminal 3 and which corresponds to plates 13: and C3 ofFigs. 4 and 5. Shaft S4 also carries an insulating bushing or sleeve 4which rotates with the shaft and on which is mounted a thick plate E forrotation with the shaft. If desired, the plate E may be hollow, or maybe two thin plates electrically connected together. Capacity C1; is thecapacity between plate A1, B1 and plate B2, C3;

and capacity Co is the capacity between plate A2, C2 and plate B3, C3.Capacity CA is made up of three components, CA in parallel with C11: andC21; in series. The component CA is the direct capacity between plateA1, B1 and plate A2, C2; the component Cu: is the capacity between plateA1, B1 and plate E, and the component 02E is the capacity between plateA2, C2 and plate E. When shaft S4 rotates plate B3, C3 away from A1, B1and A2, C2 to decrease the capacities CB and Co and increase componentCA, the shaft also rotates plate E toward plate A1, B1 and. plate A2, C2to increase the components Cu: and C211.

Any of the attenuators described above may be readily designed to give aloss in terms of voltage ratio of, for example, 40 to decibels for asingle T or 7r section and operate with a single control calibrateddirectly in terms of loss. This type of variable attenuator is cheaperto build, quicker to operate, and, particularly at high frequencies,easier to calibrate and more accurate than the resistance type ofvariable attenuators now in use. The variable condenser attenuator canbe put in series with any number of variable or fixed T or w capacityattenuators of the same image capacities and the losses of the string ofattenuators directly added.

That is, the attenuators can be connected in tandem to form a networkgroup, with the image impedances of successive networks matched at eachjunction, and with the group terminated in its image impedances, thetransfer constant of the group then being the sum of the transferconstants of the individual networks.

Fig. 7 shows an example of this in application of the attenuator tomeasurement of attenuation (insertion loss). The circuit of this figureis of the general type of that of Blackwell Patent 1,261,096, April 2,1918, for Measuring transmission loss by the comparison or substitutionmethod. An oscillator or other suitable source 5 of voltage of thefrequency for which the loss measurements are to be made, suppliescurrent through two branch circuits to a circuit which comprisesdetector 1 and milliammeter 8 and which can be connected to eitherbranch by transfer switch 9. The lower branch comprises the unknownnetwork Ii) whose attenuation is to be measured and proper terminatingimpedances H and I2. The upper branch comprises a fixed pad orattenuator l3, a zero balancing pad 14 and a calibrated attenuator I5.

The attenuator I5 is shown by way of example as a T-network of variablecapacities C2, Cb and Cc, such as that of Fig. l, and may be forinstance of the type shown in Figs. 2 and 2A or the type shown in Figs.3 and 3A; and the pad I4 may be an attenuator of similar type. Each ofthese attenuators l3 and 14 may be designed as indicated above with itsplates shaped to give a constant image capacity independent of theirangular setting. Networks l4 and 15 may have their image capacities attheir junction matched; the network [3 may match the image impedance ofnetwork l4; and condenser l6 may terminate the network [5 in its imageimpedance at its output terminals. If desired, a dial (not shown) may beconnected to the shaft (S or S) of attenuator 15, to indicate theangular setting of the plates of the attenuator, and may be calibrateddirectly in attenuation units. This attenuator may be designed to have awide variation of attenuation, for example 40 decibels or more, withadjustment by means of a Vernier if high precision is desired.Designating the input and output voltages of the upper branch circuit ofFig. 7 as V1 and V2, respectively, the insertion loss of this branch is20 log 71 and designating the input and output currents of the unknownnetwork it! as 11 and I2, respectively, as indicated by the arrows inthe figure, the insertion loss of this network is I 20 logz These twolosses are equal when the readings of the meter 8 are the same for thealternate positions of switch 9. As indicated above, a variableattenuator such as i5 is accurate over a much wider range of frequencythan a resistance type variable attenuator. The mommy of the attenuationmeasurements or circuit readings given by the circuit of Fig. 7 isindependent of frequency when the admittance of grid leak ll is madehigh compared to the capacity l6.

In making measurements, resistances or impedances H and i2 are adjustedto the values of the impedances between which the performance of thenetwork is to be determined. The unknown network is removed and theterminals connected by parallel wires to preserve the configuration ofthe line. The fixed (standard) pad is removed from the other branch ofthe circuit and the calibrated attenuator is set on zero. The switch isshifted from one position to the other and the zero balance pad isadjusted until the detector shows equal readings for the two switchpositions. The unknown network is replaced and the calibrated attenuatoradjusted (in conjunction with the fixed standard, pad, if the pad isnecessary), until the detector readings are the same for both switchpositions. The loss of. the unknown network then equals the reading ofthe calibrated attenuator plus the fixed pad.

Any of the T or ii" variable attenuators described above may be appliedto gain control in tuned circuits, for example, as illustrated by suchapplication of the T form of attenuator in the vacuum tube circuits ofFigs. 8 to 10. These vacuum tube circuits may be, for instance, tunedamplifiers or modulators, of radio sets, for example. The attenuator isshown as an element 20 of the circuit coupling tubes 2i and 22, and asworking into a capacity 23. The capacity 23 is the image capacity of theattenuator at its output end. The impedance of capacity 23 may be, ormay include, the capacitive component of the effective input impedanceof a load (such for example as vacuum tube 22) whose remaining componentof effective input impedance is high compared to the impedance ofcapacity The input capacity of the attenuator is designated Co asindicated on the drawings, and is the image capacity of the network atits input end. This capacity is independent of the attenuator setting,i. e., its loss. Therefore, this capacity can form a fixed part of atuning capacity, the remainder of which may be either fixed or variable,and the loss of the attenuator can be varied without altering the tuningcapacity or interfering with the variation of the tuning.

For example, in Fig. 8 the tuning circuit is constituted by inductance25, adjustable capacity CTZ and fixed capacity Co, all in parallel; inFig. 9 the tuning can be set by adjustable capacity C're and couplingcoil 26, the capacity Co being constant; and in Fig. 10 the tuning canbe varied by adjustable capacity GT2 which, in parallel with the fixedcapacity Co, is in series with tuning inductance 2T. ulators such asthose of Figs. 8 to 10 may be operated by first adjusting the variableportion C'r2 of the timing capacity, and then controlling the gain byadjustment of the attenuator 20. Such gain adjustment does not disturbthe tuning.

Any of. the T or 1r variable attenuators described above may be appliedto control of retroaction or feed-back in wave translating systems, forexample, as illustrated in Figs. 11 and 12 by application of the T formof attenuator to automatic gain control of amplifiers for compensatingtransmission line attenuation changes. These changes may be due forexample to variation of temperature or other weather conditions to whichthe line is subjected.

In Fig. 11 an amplifier comprising vacuum tubes 35, 32 and 33 in cascadeconnection, receives waves from incoming line or circuit L1 terminatedin amplifier input transformer T1 and transmits the amplified wavesthrough amplifier output transformer T2 to outgoing line or circuit L2.he circuits L1 and L2 may be, for example, sections of a multiplexcarrier cable or open wire circuit, the amplifier amplifyingsimultaneously the waves of a number of carrier telephone channelsand/or carrier channels, extending over a wide frequency range, forinstance, the range from 8 to 56 kilocycles per second.

The amplifier may have a forwardly transmitting path comprising thetubes 3i, 32 and 33, and a feed-back path P, and be of the general typeof negative feed-back amplifier in which waves, including those of therange of transmitted frequencies, are so fed back through the feed-backpath from the output to the input of the forwardly transmitting path asto reduce the gain of the amplifier below the value that it would havewithout feed-back, in order to reduce unwanted modulation or non-lineareffects and render the gain stability greater than it would be withoutfeed-back. That type of amplifier is disclosed, for example, in thecopending application of H. S. Black, Serial No. 606,871, filed April22, i932, for Wave translation systems, and in H. S. Blacks article onStabilized Feed-Back Amplifiers, published in Electrical Engineering,January, 1934, pages 114 to 120. Herein, as in those disclosures, adesignates the propagation of the forwardly transmitting path of theamplifier and B the propagation of the feed-back path, the quantity Bdesignating the modification that a voltage undergoes in transmissiononce around the closed feed-back loop. The quantity 13 may be largecompared to unity, as for example of the order of 50 or and ,u may belarge compared to B.

A transmission equalizing network N having a terminating resistance isshown in the feedback path P. The networkN may be, for example, anattenuation equalizer of the type of equalizer 314 shown in Fig. 65 ofthe above-mentioned copending application 606,871, or in Fig. l of theabove-mentioned Patent 1,956,547, with its attenuation-frequencycharacteristic like that of the cable or line in which the amplifier isconnected, so that the amplifier will equalize the line attenuation. Asexplamed in the above-mentioned copending application 606,871 and theabove-mentioned published article, and also in the above-mentionedPatent 1,956,547 and in British 371,887, the frequency variation of theequalizer attenuation contributes like frequency Tuned amplifiers ormodvariation to the over-all gain of the amplifier, so if theattenuation-frequency characteristic of the equalizer is made similar tothat of the line or circuit to be equalized, instead of complementary toit as in the usual case of an equalizer in a line, the equalizer tendsto compensate for the variation of the attenuation of the line withfrequency.

The over-all gain of the amplifier may be controlled by a condenserattenuator, which may be, for example, the variable attenuator of Figs.2 and 2A or that of Figs. 3 and 3A described above, comprising variablecapacities Ca, Cb and Cc, and which may have its input terminals I and 3connected across the equalizer terminating resistance 35 and its outputterminals 2 and 3 connected in series with the secondary winding of theamplifier input transformer. Its image capacities are not necessarilyvmade constant. The over-all gain of the amplifier can be varieduniformly or the same amount at each frequency, over the utilizedfrequency range, by adjusting the setting of the attenuator.

The adjustment may be made manually. However, if desired it may be madeautomatically, for example, by pilot wire or pilot channel controlequipment indicated at 36. For instance, the equipment 36 may beautomatic pilot wire transmission regulator control equipment such asthat which operates transmission regulating rheostat I2 of the systemdisclosed in the abovementioned Patent 1,956,547, May 1, 1924, or suchas that of Shackleton-Edwards Patent 1,960,350, May 29, 1934 Case 26-16;or may be automatic pilot channel transmission regulator controlequipment such as that which operates the equalizer-potentiometer 36, 31of Aifel Patent 1,511,013 October 7, 1924 Case 24, or such as that of R.W. Chesnut application Serial No. 5,696, filed February 9, 1935, forGain control circuits. The control equipment 36 may thus cause theattenuator to compensate for line attenuation changes, for example,changes produced by temperature or other weather changes to which theline is subjected.

The capacities Ca and Co form a condenser potentiometer in the feed-backpath P. Ca and Co are series and shunt elements, respectively, in thepath P. Capacity in shunt to the feed-back path P, as for example thecapacity to ground of transformer T1 appearing between 2 and 3,ordinarily introduces phase shift objectionable as tending to producesinging (for example tendency of the amplifier to sing at a frequencywell above the utilized frequency range). The capacity Cb can bedesigned to correct for (a) errors due to shunting the output of theequalizer by the condenser potentiometer formed by Ca and Ge, or (b) theerror caused by the impedance of the loop 3, (Z, G, '3 including theeffective input capacity Cg of tube 3| and the inherent capacity toground of transformer T1, which is effectively connected acrossterminals 2 and 3. With reference to (a), at high frequencies the netcapacity shunted around 35, which depends on all the capacities shown aswell as on the capacity of the transformer to ground, tends to produceundesirable phase shift. This capacity can be reduced to a minimum byproperly designing Ca, Cb and Co with reference to reducing the effectof the transformer capacity to ground. With reference to (b), Ca, Cb andCo may be designed to keep the loop impedance G-3-d attached to thesecondary winding of transformer T1 con stant. Both conditions cannot besimultaneously realized but may be approximately realized in many cases.

Even with C omitted, (for example made very large, or short circuited),the condenser potentiometer affords a desirable gain control means,advantageous especially as regards avoiding introduction of an undueamount of phase shift in the feed-back path. With Cb omitted, (and thesum of the impedance of transformer T1 and the impedance of capacity Cg,in series, large compared to the combined impedance of the capacity Ccand the parallel connected capacity to ground of the transformer), thephase shift in the feed-back path will be less with the condenserpotentiometer than with a resistance potentiometer or a variableresistance shunted across the equalizer output for gain control, as longas the equivalent capacity of Ca in series with the two parallelcapacities Co and the capacity to ground of transformer T1, is less thanthe capacity shunted across the equalizer output (including the capacityto ground of the transformer) when the gain control is obtained by theresistance potentiometer or the shunt resistance. Moreover, to theextent that the capacity Cc is made to include the capacity to ground ofthe transformer, the reduction in phase shift can be improved.

Fig. 12 shows a system like that of Fig. 11 except that the network N isreplaced by an attenuation equalizing network N and the condenserattenuator comprising the capacities Ca, Cb and Co has its inputterminals 1 and 3 connected across the series arms of the equalizer andits output terminals 2 and 3 connected in series with the equalizerterminating resistance 35 and the amplifier input.

The capacities Ca and Cc form a condenser potentiometer having inputterminals l and 3 and output terminals d and 3. This condenserpotentiometer forms with the equalizer N and its terminating resistance35, a potentiometer equalizer having as its input terminals the inputterminals of the equalizer N and having output terminals d and 39. Aspointed out in the copending application of C. R. Eckberg, Serial No.5,717, filed February 9, 1935 for Transmission regulating systems, thepotentiometer of a potentiometer equalizer of this general type can beadjusted to obtain, between the input terminals and the output terminalsof the potentiometer equalizer, a voltage attenuation that is anydesired fraction of the total voltage attenuation of the equalizer N.Thus, varying the setting of the potentiometer varies the attenuation ofthe potentiometer equalizer so as to give the same attenuation changethat would be obtained from the network N by constructing the network ofa large number of similar sections of very small attenuation per sectionand varying, one by one, the number of tandem connected sectionsincluded in circuit.

The network N may be so designed that the frequency variation of itsattenuation simulates the frequency variation of the difference betweenthe maximum line attenuation (corresponding for example to the linesattenuation-frequency characteristic for maximum line temperature), andthe minimum line attenuation, (corresponding for example to the linesattenuation-frequency characteristic for minimum line temperature). Thatis, the networks attenuation-frequency characteristic may simulate thecharacteristic that represents the difference between the lines highestand lowest attenuation-frequency characteristics. Then, with Cs omitted(or made very large or short-circuited), the setting of thepotentiometer can be varied to vary the over-all gain of the amplifierin such manner as to compensate for the variation of the lineattenuation due to the temperature changes or other weather changes towhich the line is subjected.

The addition of the condenser Cb serves the same purpose as in the caseof the system of Fig. 11. Moreover, it increases the flexibility of theequalizer. That is, with three variable elements in the system the lossof the equalizer may be varied as the loss of a series of smallequalizers might be and a flat loss independent of frequency added inamount having any desired relation to the amount of equalization asrepresented by the number of the small equalizers that would be requiredin circuit to produce such amount of equalization.

What is claimed is:

1. A three-terminal network composed of three capacities Ca, Cb and Coforming three separate arms, and means for so varying the componentcapacities as to produce a prescribed variable relation of theattenuation between two pairs of the terminals as input and outputterminals and the image capacities of the network, capacity Ca havingthe value c /G sinh e /C cosh 0- /C capacity Cb having the value C n Fsinh 9 cosh 0- /C and Cc having the value 1/ 01 2 si nh where C01 andC02 are the image capacities of the network and 6 is the real part ofthe attenuation constant of the network expressed in nepers.

2. A three-terminal network as set forth in claim 1 with difierent imagecapacities at the two ends of the network.

3. A three-terminal network as set forth in claim 1 with equal imagecapacities at the two ends of the network.

4. A three-terminal, three-capacity network of capacities Ca, Cb and Cc,and means for so varying all of the capacities simultaneously as to varythe attenuation of the network between two pairs of the terminals asinput and output terminals and at the same time maintain the imagecapacities of the network constant, capacity Ca having the value Con/602 sinh 0 /0 cosh 0- /C capacity Cb having the value co /E01 sinh 0 {GTcosh 9 /C and C0 having the value 1/ cg cog sinh 0 where C01 and C02 arethe image capacities of the network and e is the real part of theattenuation constant of the network expressed in nepers.

EDWARD B. PAYNE.

